The production of light which as far as possible is unpolarized is desired in an illumination system of a microlithographic projection exposure apparatus for many uses. In general, the light emitted by the corresponding laser source is linearly polarized light. Accordingly, the linearly polarized light should be depolarized. To the extent that there is residual polarization, this can lead to effects as a consequence of polarization-dependent transmission or reflection properties of mirrors and because of AR-layers which are present on lenses and which are dependent in respect of their strength and the preferred polarization direction which is respectively obtained, on the illumination setting which is used.
An illumination system is known which includes an arrangement for modifying the degree of polarization, which in the direction of light propagation in succession has a lambda/4 plate, a lambda/2 plate and wedge-shaped deflection prisms which are freely rotatable in order to achieve a variation in the degree of polarization of the illumination light between a linear polarization state and an unpolarized state, via a change in the orientation of the optical crystal axes.